Support region padding for MPU programming

[thejpster]

Make it easier to set up the MPU

• Add an API to get the various linker output sections (aarch32_rt::region)
• Adjust link.x to allow for adjustable alignment of regions
• Adjust link.x to allow for adjustable padding between stacks
• Add an example that programs the MPU

The MPS3-AN536 padding is set at 64K, so that the normal build and the +d32 build end up with the same memory map (otherwise their output for the mpu_region example would be different).

[wt]

Everything I didn't mention looks okay assuming all the magic numbers in the reference files are correct.

[wt]

I'm not comfortable that future additions to the Region enum will not get caught if they aren't added to this array. I think that it would be better to have a RegionIter type that implements iterator and makes sure that all the values are in the iterator with an exhaustive match on the current section.

Something like:

struct RegionIter {
    current: Region;
};

impl Iterator for RegionIter {
    type Item = Region;

    fn next(&mut self) -> Option<Self::Item> {
        let next = match self.current {
            Region::VectorTable => Region::Text,
            Region::Text => Region::Rodata,
            // etc...
            Region::Uninit => return None,
        }
        self.current = next
        Some(next)
    }
}

iter() should return this initialized RegionIter.